The preparation of polyether carbonate polyols by catalytic reaction of alkylene oxides (epoxides) and carbon dioxide in the presence of H-functional starter substances (“starters”) has been the subject of intensive study for more than 40 years (e.g. Inoue et al., Copolymerization of Carbon Dioxide and Epoxide with Organometallic Compounds; Die Makromolekulare Chemie 130, 210-220, 1969). This reaction is shown in schematic form in scheme (I), where R is an organic radical such as alkyl, alkylaryl or aryl which may in each case also contain heteroatoms, for example O, S, Si, etc., and where e, f, g and h are each integers, and where the product shown here in scheme (I) for the polyether carbonate polyol should be understood as meaning merely that blocks having the structure shown may in principle be retained in the polyether carbonate polyol obtained but the sequence, number and length of the blocks and the OH functionality of the starter may vary and is not restricted to the polyether carbonate polyol shown in scheme (I). This reaction (see scheme (I)) is highly advantageous from an environmental standpoint since this reaction comprises converting a greenhouse gas such as CO2 into a polymer. Further products, actually by-products, formed here are the cyclic carbonate shown in scheme (I) (propylene carbonate when R=CH3 for example) and dioxanes (dimethyldioxane when R=CH3 for example). The dioxanes are also formed when the polyether carbonate polyols are subjected to thermal exposure. A characteristic feature of polyether carbonate polyols is that they contain ether groups between the carbonate groups.

EP-A 0 222 453 discloses a process for preparing polycarbonates from alkylene oxides and carbon dioxide using a catalyst system composed of DMC catalyst and a cocatalyst such as zinc sulfate. This polymerization is initiated by one-off contacting of a portion of the alkylene oxide with the catalyst system. Only thereafter are the residual amount of alkylene oxide and the carbon dioxide metered in simultaneously. The amount of 60% by weight of alkylene oxide compound relative to the H-functional starter compound, as specified in EP-A 0 222 453 for the activation step in examples 1 to 7, is high and has the disadvantage that this constitutes a certain safety risk for industrial scale applications because of the high exothermicity of the homopolymerization of alkylene oxide compounds.
WO-A 2003/029325 discloses a process for preparing high molecular weight aliphatic polyether carbonate polyols (weight-average molecular weight greater than 30 000 g/mol), in which a catalyst from the group consisting of zinc carboxylate and multimetal cyanide compound is used, this catalyst being anhydrous and first being contacted with at least a portion of the carbon dioxide before the alkylene oxide is added. Final CO2 pressures of up to 150 bar place very high demands on the reactor and on safety. Even the extremely high pressure of 150 bar resulted in incorporation of only about 33% by weight of CO2 up to a maximum of 42% by weight of CO2. The accompanying examples describe the use of a solvent (toluene) which has to be removed again by thermal means after the reaction thus resulting in increased time and cost demands. Furthermore, the polymers, with a polydispersity of 2.7 or more, have a very broad molar mass distribution.
WO-A 2008/058913 discloses a process for preparing flexible polyurethane foams exhibiting reduced emissions of organic substances, wherein the polyether carbonate polyols employed preferably have a block of pure alkylene oxide units at the chain end.
EP-A 2 530 101 discloses a process for preparing polyether carbonate polyols in which at least one alkylene oxide and carbon dioxide are reacted onto an H-functional starter substance in the presence of a DMC catalyst. However, EP-A 2 530 101 does not disclose how polyether carbonate polyols may be stabilized toward thermal exposure in order to achieve a very low dioxanes content after thermal exposure.
U.S. Pat. No. 4,145,525 discloses a process for thermal stabilization of polyalkylene carbonate polyols. The polyalkylene carbonate polyols disclosed in U.S. Pat. No. 4,145,525 contain alternating units of alkylene oxide and carbon dioxide. U.S. Pat. No. 4,145,525 teaches reacting at least some of the terminal hydroxyl groups of the polyalkylene carbonate polyol with a phosphorus compound reactive toward hydroxyl groups to form an oxygen-phosphorus compound. U.S. Pat. No. 4,145,525 does not disclose polyether carbonate polyols. However, U.S. Pat. No. 4,145,525 does not teach those skilled in the art how polyether carbonate polyols having a very low dioxanes content may be prepared by addition of one or more alkylene oxides and carbon dioxide onto one or more H-functional starter substances in the presence of a double metal cyanide catalyst.
It is an object of the present invention to provide a process for preparing polyether carbonate polyols, wherein the process affords a product which has a very low dioxanes content after thermal exposure. It is a particular object of the invention to provide polyether carbonate polyols which not only exhibit a very low dioxanes content after thermal exposure but are also suitable for preparing flexible polyurethane foams.